Conventionally, in a can cover for a positive pressure can of, e.g., beer or a carbonated beverage, there are proposed various lightweight pressure-resistant can covers each in which a pressure resistance is enhanced and reductions in material and the thickness of the material are achieved by reducing the center panel diameter of the can cover and changing the shape of a chuck wall (see Patent Documents 1 to 3).
As a typical example, in a can cover according to the invention of Patent Document 1, a can cover diameter d1/a panel diameter d5=1/0.717 is established, the center panel diameter is small as compared with that of the conventional pressure-resistant can cover, and the pressure tightness is enhanced by setting the inclination angle c of the chuck wall to about 43° which is extremely large correspondingly to the small center panel diameter. When the can cover of this type is seamed with a can body filled with a content, the can cover is supplied to the can body by a can cover transport turret and seamed therewith. However, as shown in FIG. 6 as Comparative Example described later, when the can cover is supplied to the can body, the chuck wall of the can cover is positioned close to the top of a flange of the can body, its eccentricity amount is large in a can cover shift section to the can body before an intake center p described later is reached, and the displacement of the center position tends to occur, and hence centering or attachment of the can cover to the can body is not properly performed, and an accident that double seaming is performed in a mismatch state in which the can cover is eccentric relative to the can body tends to occur.
That is, in the line of seaming the can cover with the can body, conventionally, a travel center line M of the can cover by a can cover supply turret intersects a travel center line L2 of a can body transport conveyer and a travel center line N of a lifter plate (a knock-out pad of a seaming head is present on the same axis) at the intake center p on a line which joins the center of the can cover supply turret and the center of a seaming turret, as shown in FIG. 5. Accordingly, a can body c, a can cover e, and their respective center lines are set so as to substantially overlap each other in the can cover shift section to the can body (a region indicated by an angle α in FIG. 3), and the attachment of the can cover is performed at the intake center p. At this point, in the can cover shift section before the intake center p described above, there are cases where the can cover is supplied in an eccentric state in which the center position of the can cover is displaced with respect to the center position of the can body.
In the can cover proposed in Patent Document 1 in which the surface of the center panel is small and the inclination angle of the chuck wall is large, as shown in FIG. 6, the point of tangency between the chuck wall of the can cover and the flange of the can body is present on a substantially upper surface of the flange, and the displacement of the center position of the can cover with respect to the center position of the can body is conspicuously seen in the can cover shift section before the intake center p where the attachment of the can cover to the can body is performed. As a result, the displacement occurs, the eccentricity amount is increased, and the placement width of the top of the curled portion of the can cover which is placed on the flange of the can body is increased. When seaming is performed in a state where the top of the curled portion of the can cover is placed on the flange of the can body, as shown in FIG. 8, the curled portion of the can cover cannot be seamed with the flange of the can body normally, and a seam failure called a false seam in which a curled hook CH is crushed on a body hook BH tends to occur. As shown in the drawing, the false seam is hidden inside a seamed portion so that it is difficult to locate the false seam from the outside, and the occurrence of the false seam is a problem which should be avoided in terms of quality control.
Note that the detail of the cause of the increase in the eccentricity amount when the conventional lightweight pressure-resistant can cover proposed in Patent Document 1 is seamed is described later.
In addition, in each of can covers proposed in Patent Documents 2 and 3, the above-described problem is reduced, but a sufficiently satisfactory solution to the problem is not achieved yet.
Patent Document 1: Japanese Translation of PCT Application No. H11-505791
Patent Document 2: Japanese Patent Application Laid-open No. 2006-122990
Patent Document 3: Japanese Patent Application Laid-open No. 2010-215274